1. Field of the Invention
This invention relates to technology for detecting the peak position of an analog signal, and in particular relates to technology for accurately detecting the peak position even when the amplitude of the analog signal changes.
2. Description of the Related Art
In recent years, various projectors have been proposed having a resonance circuit made from a coil and a capacitor for lamp control of the discharge lamp. With this kind of projector, the discharge lamp is connected in parallel with the resonance circuit capacitor, and by controlling the frequency of the voltage applied to the resonance circuit to be the resonance frequency of this resonance circuit, applies the necessary voltage for discharge to the discharge lamp connected to the resonance circuit and lights it. Note that as this kind of projector, there is the projector disclosed in JP05-217682A.
With a projector equipped with the resonance circuit for discharge lamp lighting control, there are cases when the resonance frequency of the resonance circuit changes according to changes in the discharge lamp discharge gap friction, the discharge lamp temperature characteristics or the like. In such cases, if the frequency of the voltage applied to the resonance circuit is fixed, it is not possible to apply the necessary voltage to the discharge lamp, and the discharge lamp will not light. In light of this, to maintain the lit state even when the resonance frequency changes, there was a desire to change the frequency of the applied voltage according to changes in the resonance frequency. In response to this kind of demand, proposed was a projector for which the frequency of the voltage applied to the resonance circuit is changed based on the current value of the resonance circuit. Following, we will briefly describe the operation of the lighting control of this kind of projector.
With a constitution with the discharge lamp connected to the resonance circuit, when the frequency of the voltage applied to the resonance circuit is gradually increased and approaches the resonance frequency, discharge starts at the discharge lamp, and a large current flows to the resonance circuit. In this case, the current value of the resonance circuit increases together with the increase of the applied voltage frequency, and becomes the maximum value at the resonance frequency. In light of this, with this projector, the current value at the resonance circuit is detected by a current sensor, and when the detected current value is a specified value or greater, the applied voltage frequency is increased or decreased so as to maintain a specified phase difference between the phase of the resonance circuit current and the phase of the applied voltage. By working in this way, even when the resonance frequency changes, by controlling the applied voltage frequency so as to maintain the state of the current flowing at a specified value or greater to the resonance circuit, it is possible to maintain a stable lighting state.
With this kind of projector, it is necessary to detect the phase difference by comparing the resonance circuit current phase and the applied voltage phase, but this comparison is preferably a comparison of the respective peak level positions (following simply called “peak position”). However, it is difficult to detect the peak positions when the amplitude of the current or the voltage of the resonance circuit is varied.
The problem described above is not limited to the current and the voltage of the resonance circuit, but can also occur when trying to detect the peak positions for analog signals for which the amplitude can change.